Auto merge of #113339 - lqd:respect-filters, r=tmiasko

Filter out short-lived LLVM diagnostics before they reach the rustc handler

During profiling I saw remark passes being unconditionally enabled: for example `Machine Optimization Remark Emitter`.

The diagnostic remarks enabled by default are [from missed optimizations and opt analyses](rust-lang/rust#113339 (comment)). They are created by LLVM, passed to the diagnostic handler on the C++ side, emitted to rust, where they are unpacked, C++ strings are converted to rust, etc.

Then they are discarded in the vast majority of the time (i.e. unless some kind of `-Cremark` has enabled some of these passes' output to be printed).

These unneeded allocations are very short-lived, basically only lasting between the LLVM pass emitting them and the rust handler where they are discarded. So it doesn't hugely impact max-rss, and is only a slight reduction in instruction count (cachegrind reports a reduction between 0.3% and 0.5%) _on linux_. It's possible that targets without `jemalloc` or with a worse allocator, may optimize these less.

It is however significant in the aggregate, looking at the total number of allocated bytes:
- it's the biggest source of allocations according to dhat, on the benchmarks I've tried e.g. `syn` or `cargo`
- allocations on `syn` are reduced by 440MB, 17% (from 2440722647 bytes total, to 2030461328 bytes)
- allocations on `cargo` are reduced by 6.6GB, 19% (from 35371886402 bytes total, to 28723987743 bytes)

Some of these diagnostics objects [are allocated in LLVM](rust-lang/rust#113339 (comment)) *before* they're emitted to our diagnostic handler, where they'll be filtered out. So we could remove those in the future, but that will require changing a few LLVM call-sites upstream, so I left a FIXME.

Author: bors